# Set voltages and amperage for devices

I see some devices have voltage and amperage ratings on them. Are these just the recommended operating conditions? Or are they something else?

Also, how do engineers determine how much voltage and current does a circuit need?

I see ratings for resistivity as well. Why does resistivity matter in, say, a vacuum cleaner?

Related Electrical Engineering News on Phys.org
Integral
Staff Emeritus
Gold Member
Circuits involving solid state devices have voltage requirements that are determined by the manufacture of the device. Often these voltages are industry standards.

Examine any of the wall warts that you may have for your various electronic devices. You will find that they have a specified input voltage, like 120VAC, or now frequently they say 230 or 120VAC. You never want to exceed the stated input voltage this is a very quick way to destroy your wall wart. Fortunately this is not easy to do since anything other then these voltages is uncommon. Be careful about taking US products to Europe.

Each wall wart also will specify a output voltage and current. These need to be compared to the input voltage and current as stated on your electronic device. It is risky business to connect a voltage higher then a device is rated for. So if your boom box is rated at 6V input connecting a 12V output wall wart could very well destroy your boom box. Once you have matched the voltage check the current ratings on both the wall wart and the device. The wall wart current rating must be equal to or greater then the current rating of the device. Connecting a lower current wall wart can damage your wall wart.

I see some devices have voltage and amperage ratings on them. Are these just the recommended operating conditions? Or are they something else?
These are for ideal conditions but shouldn't vary too much. Here is Australia our electrical supply system specifies 230V although the service rules allow +10% to -2% for supply to the house. This means you could have between 225V and 253V depending on how far from the transformer you are. Lets look at an example to see how much difference it makes.

An easy one is a hot water system rated at 3.6kW 230V to work out current 3600/230= 15.65A
next work out the resistance of the heating coil R = V/I 230/15.65 = 14.7 ohms
now we can work out how many amps the same hot water system would draw for different possible voltages. I=E/R

225V 225/14.7= 15.3A
253V 253/14.7= 17.2A

As you can see the same device can have its current vary by about 10% depending on voltage. Nothing really to worry about but I hope it answers your question. The plate ratings are for ideal circumstances.

I see ratings for resistivity as well. Why does resistivity matter in, say, a vacuum cleaner?
Haven't really seen that except for larger motors. You can test the winding resistance to see if there are any shorts etc... Anyone else?